Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus having an illumination system and a projection objective. In that case the image of a mask (=reticle) illuminated by the illumination system is projected by the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective to transfer the mask structure onto the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range (wavelengths of for example about 13 nm or about 7 nm) due to the lack of availability of suitable translucent refractive materials. Particularly in high-aperture projection objectives (for example with values of the numerical aperture of greater than 0.4) a small spacing (for example in the range of 20-50 mm) is used between the mirror arranged most closely to the wafer and the wafer, which in turn involves a comparatively thin mirror (for example the ratio of maximum diameter to maximum thickness, with respect to the surface which is optically used or acted upon light, can be 10 or more).
With thin mirrors of that kind, the increase in mirror dimensions (involved with increasing numerical apertures) to comply with desired specifications makes it desirable to use increasing natural frequencies to rigidly maintain the positions of the optical elements relative to each other even when external vibrations occur while also providing adequate stiffness with respect to parasitic and mass-dependent moments or interference forces (generally produced by screwing of the mirrors after polishing).
Approaches for resolving issues related to applying mechanical stresses to optical elements such as mirrors or lenses are known, for example, from WO 2005/054953 A2 or US 2009/0122428 A1.